I've looked at all the different plat proposals and here is what I found:

Assassin's Creed II is the plat earned by most people. Astonishing 4382 different players has the plat. Followed by Uncharted 2 with 3938 and InFAMOUS with 3447.
But this doesn't necessarily mean that these plats are the most popular plats. To get a better picture I also looked at the percentage of players owning the plats per players owning the games.
In this list Assassin's Creed II takes a 4th place. First place goes to Sly Cooper and the Thievius Raccoonus with 80,92% Followed by Spare Parts with 74,09% and Sly 2 with 73,42%.

Other games just don't touch AC2, because it's a popular game with an easy platinum. But as an overall series, Uncharted is the platinum king with over 110,000 combined. GoW series is close second with just over 100,000 with 5 GoW games, CoD is probably third with about 80,000 plats over the 5 CoD games. ACB and ACR only amount to a combined total of around 13,000 plats, so the AC series as a whole is probably 4th at around 70,000.